This invention relates to color cathode-ray tube apparatus with a shadow mask.
In a color-cathode ray tube, the requisite color is achieved by selectively activating phosphors which emit, respectively, three different primary colors, red, green and blue. Three electron beams are generated which are intended to activate respectively, the red, green and blue phosphors. The primary color components of the required color are obtained by appropriately modulating the respective beams. The problem of ensuring that a beam lands only on the appropriate phosphor, that is, of ensuring color purity, is solved by the shadow mask. This is a parallax device located close to the screen of the cathode-ray tube and comprises an apertured metal sheet, the apertures being arranged such that a beam can land only on the appropriate phosphor. Usually, the shadow mask is used to control the deposition of the phosphors on the screen.
There are two patterns of shadow mask in general use. In the first, the dot shadow mask has about 350,000 circular holes arranged in a hexagonal array. The individual electron beams are large enough to cover about three holes, so that at any given time the red gun, for example, has the potential to activate several phosphor dots. To the viewer of the cathode-ray tube screen, what appears as a single red patch is actually the emissions from several phosphor dots. The other type of shadow mask is slotted and consists of vertical rows of slots, each vertical row being offset relative to the adjacent rows. With each slot of the shadow mask are associated three slot shaped different color phosphors. In a variation of this type of shadow mask, known as the aperture grill, the slots extend from the top to the bottom of the display area of the screen to form continuous slits. Heretofore, slotted shadow masks have been used in conjunction with a horizontal line raster in which the electron beams are caused to trace a sequence of parallel lines extending from side to side of the screen. The lines of the raster are perpendicular to the slots or slits in the shadow mask.
The contemporary shadow mask color television tube gives a satisfactory picture when used in domestic surroundings with the screen being viewed from a distance of 2 or 3 meters. Using a shadow mask color television tube as a display terminal for the display of text or of complex graphics where the user is at a distance of 1 meter or less from the screen imposes higher standards of resolution on the color tube, not only because at this distance the eye can more readily resolve the picture into its components, but also because of the large amount of information it is desirable to present. The typical aim of a designer would be to construct a terminal capable of displaying a picture consisting of 512 rows, each containing 720 picture elements (pels). A picture element is the smallest independently controllable region of the screen at which any color can be displayed. To reduce brightness variations due to random displacements of the electron spots relative to the shadow mask apertures, it is generally accepted that there should be at least 1.4 times as many apertures per row as there are pels. This implies the provision of about 1000 apertures per row. Standard television shadow masks now available have about 500 apertures per row. Substantial improvements in manufacturing techniques will be required to achieve the required standard at reasonable cost.
Another problem is encountered when the number of apertures is increased. It is usual to provide a purity margin between the different phosphor regions to allow for relative displacements of the beam sources, the mask and the screen, which occur for example when the electron guns are installed or the mask heats during use. The purity margin permits some misregistration of these items without activating an incorrect phosphor or partially activating a desired phosphor, resulting in loss of color purity. This can be achieved by making the cross-section of an electron beam emerging from a shadow mask aperture either substantially smaller or substantially larger than the area of the corresponding phosphor dot or stroke. The former case is termed positive tolerance and the latter case negative tolerance. Current design practice uses negative tolerance. In this case the purity margin may be separated into a `leaving` tolerance and a `clipping` tolerance. The former is defined as the smallest distance in any direction which the mask can move relative to the screen before part of a phosphor dot is no longer illuminated; the latter is measured in the same way at the point where electrons start to illuminate an adjacent (incorrect) phosphor dot. In the following discussion, the leaving tolerance will be assumed equal to the clipping tolerance, and the term "purity margin" will be used to describe either tolerance. As the number of apertures and thus the number of phosphor elements increases, the relative area of the screen occupied by the purity margin increases and the number of electrons getting through the apertures and doing useful work in producing light by impacting phosphor decreases.